The present invention relates, in a first aspect thereof, to a method for manufacturing an optical fiber from a preform, comprising the steps of:
a) heating at least one end portion of said preform;
b) drawing an optical fiber from a free end of said heated end portion along a fiber drawing axis;
c) coating said optical fiber with a suitable coating material;
d) applying to said coated optical fiber a torque about said fiber drawing axis,
e) winding said coated optical fiber onto a collecting spool.
More specifically, this invention relates to a method for manufacturing an optical fiber having low PMD for use in telecommunication field.
The present invention also relates to a device for applying a torque to an optical fiber about its drawing axis, as well as to an apparatus for carrying out the above-mentioned method.
As is known, in signal transmission systems using optical fibers, above all in those systems operating over long distances, the need arises of minimizing any kind of attenuation or dispersion of the signals, in order to assure high transmission and reception standards. In particular, the need arises of minimizing a specific dispersion phenomenon known as PMD (Polarization Mode Dispersion), that implies a restriction in the width of the signal transmission band and, accordingly, a worsening in the performances of the optical fibers through which the above-mentioned signals are transmitted.
As is known, the fundamental propagation mode of a signal in an optical fiber may be seen as the resultant of two linearly polarized waves on orthogonal planes. Theoretically, in a perfectly symmetric circular section fiber (i.e., in monomode fibers) the propagation constants of the two components are identical and therefore the mode is propagates unaltered and according to a cylindrical symmetry along the fiber itself, with the two components traveling at the same speed. In contrast thereto, however, optical fibers possess in practice structural and geometrical irregularities that, by altering the above-mentioned cylindrical symmetry, bring about asymmetric stress conditions in the fibers themselves and anisotropy in their optical properties; accordingly, the two mode components meet zones with different refraction index and travel with different propagation speeds, thus causing the PMD phenomenon.
A parameter of particular importance in this respect, is the so-called coherence length or fiber beat length, that is, the necessary length of fiber needed for the two components of the fundamental mode to get in phase again with one another.
Other than by structural and/or geometrical defects which are intrinsic in the fiber, such as, for example, the presence of not perfectly circular cores, the PMD phenomenon may also be originated by asymmetric stress conditions caused by outer stresses such as, for example, those generated in operation during the spool winding or wiring steps.
It has been experimentally shown that it is possible to reduce the PMD of an optical fiber by submitting the latter to suitable external stresses during the drawing process, in particular by applying a torque to such an optical fiber.
To this end, various devices and methods for manufacturing an optical fiber having low PMD have been proposed in the past, the earlier ones based upon the technique of rotating the preform (a technique which was soon abandoned due to the evident technological difficulties and to the impossibility of obtaining high rotational speed), the most recent ones based upon the technique of applying during the drawing process a torque to the fiber about its axis (fiber spinning/twisting).
Therefore, for example, U.S. Pat. No. 4,504,000 (Thomsonxe2x80x94CSF) discloses a method for manufacturing an optical fiber having a chiralic structure with high circular birefringence, wherein an equipment provided with three pulleys applies to a fiber produced from a heated portion of a preform a torque about the axis thereof. The applied torque is subsequently xe2x80x9cfrozenxe2x80x9d in the fiber structure by means of a suitable coating film, in glass or glass-ceramic, applied on the fiber itself in a dedicated coating station.
A drawback associated to the above disclosed method is connected to the high risk of damaging the fiber surface due to the fact that the latter gets in touch with the above mentioned pulleys before being properly protected by a suitable coating film.
In order to overcome the above mentioned drawback it has been proposed, as disclosed in U.S. Pat. Nos. 5,298,047 and 5,418,881 (ATandT), to arrange the device adapted to apply the torque to the fiber downstream of the coating station. In particular, in the above mentioned patents, torque is applied by means of a fiber guiding roll having a rotation axis which extends perpendicularly to the drawing axis of the fiber and which is alternatively canting in clockwise and counterclockwise direction.
Although substantially achieving the object of reducing the PMD of the fiber, the Applicant has however noticed that the above disclosed method shows a series of drawbacks to which no adequate solution has been given up to now.
A first drawback observed by the Applicant is connected to the need of limiting the canting frequency of the guiding roll in order to avoid that, during the drawing process, a relative sliding between the optical fiber and the roll could take place. Actually, there are two reasons for such a sliding to be particularly disadvantageous: firstly because it could cause a mechanical abrasion of the optical fiber surface and, accordingly, a worsening of the mechanical resistance properties and of the performances of the fiber itself; secondly, because it does not allow the torque to be imparted to the optical fiber according to a desired law of variation, thus limiting the advantageous effects given by the applied twisting regarding the reduction of the PMD phenomenon.
The limitation of the canting frequency of the guiding roll implies, furthermore, a corresponding limitation in the drawing speed of the fiber (the two speeds being unavoidably related to one another in order to apply the desired torque to the fiber) and, therefore, a limitation in the amount of fiber produced per unit of time.
A further drawback associated with the above disclosed method is connected to the fact that the oscillations of the fiber during the drawing may induce displacements of the drawing axis which cause undesired fluctuations in the diameter of the fiber produced or unevenness and/or imperfect coaxiality between the fiber and the coating layer applied thereon.
Furthermore, such method does not allow to apply a continuous unidirectional rotation to the optical fiber, because the device which is used necessarily imparts an alternate twisting to the optical fiber.
Other methods are known for the manufacture of optical fibers, comprising the step of applying a torque to the fiber about its drawing axis and xe2x80x9cfreezingxe2x80x9d the applied torque by means of a suitable coating film before the fiber reaches the twisting means are also disclosed by Japanese Patent Application No. JP 58-020746 and by German patent DE 3010005.
JP 58-020746 discloses a method for the manufacture of optical fibers adapted to maintain a single mode circular polarization of the fiber, that is a method wherein the plane of polarization is uniformly rotated across the length of the fiber itself. In order to produce the birefringence required for the circularly polarized mode, the fiber is drawn from the free end of the preform by drawing means located upstream of the coating station and the twist is imparted to the fiber by means of a collecting spool located downstream of a coating station.
Similarly, German Patent DE 3010005 discloses a method for the manufacture of twisted optical fibers wherein the fiber is drawn and twisted by means of a collecting spool located downstream of a number of coating stations.
According to the above disclosed methods, the twist is however imparted to the solidified fiber in the region between said drawing means and said collecting spool, being thus prevented from reaching the softened bottom end of the preform.
A further drawback associated to the above disclosed method is connected to the high mass and inertia of the collecting spool used for imparting the torque to the fiber, which does not allow to increase the rotation speed of the spool (and hence the drawing speed of the fiber), as well as to reliably apply a desired law of torque variation to the fiber. Besides, there is the need of translating the spool (or providing suitable means) for allowing the fiber be collected thereon.
Finally, Japanese Patent Application No. JP 06-239642 discloses a method for the manufacture of optical fiber bundles, wherein a twist is imparted to a number of fibers in order to obtain helically twisted fibers which, once they are collected together to form a bundle, have ends randomly positioned at the incident and emergent sides of the bundle. The twist is imparted, for example, by a caterpillar located upstream of the collecting spool; the caterpillar is also used to draw the fiber from the preform.
The object of the above disclosed method is quite different from the one of the present invention. In fact, for achieving the above mentioned object, fibers having a helical extension along the axial direction are manufactured.
Also in this case and because of the high mass and inertia of the caterpillar used for imparting the torque to the fiber, however, it is not possible to increase the rotation speed of. the caterpillar (and hence the drawing speed of the fiber) as well as to reliably apply to the fiber a desired law of torque variation.
The applicant has now discovered a method and an apparatus that allow the manufacture of an optical fiber having low PMD, while overcoming at the same time the above mentioned drawbacks of the cited prior art.
According to a first aspect thereof, the present invention relates to a method of the type mentioned hereinabove, which is characterized in that the application to the above mentioned coated fiber of the torque about the drawing axis of the fiber is carried out by means of a pulley which is supported upstream of the collecting spool and rotated about the drawing axis of the fiber and on which said optical fiber is wound up with an angle substantially equal to at least 360xc2x0.
Advantageously, the method of the present invention allows to achieve the desired fiber twisting without compromising its properties of mechanical resistance and, in the meantime, allows to obtain a high drawing speed for the same, thus increasing the production of optical fiber per unit of time. Differently from the known methods, in fact, in the method according to the present invention the fiber is wound up for at least about 360xc2x0 and in substantial absence of sliding onto a member adapted to apply the desired twisting to the fiber itself (pulley); this allows to increase the rotation speed of the pulley itself (and hence the drawing speed of the fiber) without incurring in the risk of having relative sliding between the pulley and the fiber. Further on, the absence of such sliding allows to apply to the fiber the desired law of torque variation.
Advantageously, the absence of any form of oscillation also allows to avoid any interference with the step of applying the coating layer onto the fiber, whereby the layer is thus homogeneously distributed over the entire surface of the fiber.
The above mentioned pulley may be rotated about the drawing axis of the optical fiber according to a unidirectional movement, with a constant or varying rotation speed, such as by way of example a speed varying according to a sinusoidal pattern from a maximum value down to a minimum value, which may eventually be equal to zero.
Alternatively, the above mentioned pulley may be rotated about the drawing axis of the optical fiber according to an alternate movement, in clockwise and counterclockwise direction, respectively. In this instance as well, it may be foreseen a constant rotation speed, by applying such speed first according to a spin direction, then reversing the spin direction and applying the same speed value in the opposite direction. A varying speed pattern will instead consist of a speed variation, for example according to a sinusoidal pattern, from a maximum value in one direction to a maximum value in the opposite direction, passing through a zero speed. value at the point in which the inversion of rotation takes place.
Advantageously, an alternating rotation speed is applied to the pulley. This allows to prevent the presence of residual torsions on the fibers wound onto the collecting spool (i.e. the collected fiber is substantially torsion-free), thus making easier both the unwinding and wiring operations of the same.
In the following description and in the appended claims, the term: xe2x80x9ctwisting pitchxe2x80x9d is used to indicate the linear distance measured along the outer surface of the fiber between two sections thereof to which the same rotation pattern is imparted.
Preferably, the optical fiber is drawn from the preform in a way known per se, though it is in this case possible to apply a drawing speed of the fiber considerably higher than that achievable by the devices of the prior art. This allows to achieve a considerable increase of optical fiber production per time unit. Compatibly with the other components in the drawing system, with the method of the present invention it is in fact possible to apply a drawing speed of, by way of example, 15 m/sec, 20 m/sec, 25 m/sec or even 30 m/s, without any undesired effects on the twisting of the fiber.
In any case, it should be noted that the above mentioned limit value for the drawing speed of the fiber is related to the drawing techniques actually employed, and is not determined by the structural features of the twisting device whereon the pulley is installed.
In a second aspect thereof, the present invention relates to a device for applying to an optical fiber, drawn at a predetermined speed from a heated end of a preform, a torque about a fiber drawing axis thereof.
Advantageously, the device of the present invention is particularly simple from a constructional point of view and has a low cost.
Preferably, the driving means of the device of the present invention is adapted to rotate the pulley about the fiber drawing axis both in a unique direction, and in clockwise and counterclockwise direction, alternately.
Preferably, the above mentioned driving means of the pulley comprises a motor-driven fork-shaped supporting member, having a rotation axis coincident with the fiber drawing axis and whereon the pulley is pivoted in an offset position, in such a way as to be substantially tangent to said axis. The drawing directions of the fiber immediately upstream and downstream of the pulley are therefore identical, thus assuring the absence of misalignment conditions that cause undesired fluctuations in the diameter of the fiber produced and unevenness in the applied coating film.
Advantageously, the driving means of the pulley comprises an electric motor kinematically connected to the pulley, i.e., by means of a belt transmission system.
Advantageously, the reduced weight and the small size of the pulley limit the magnitude of the inertial forces acting on the device during twisting; in such a way, it is possible to achieve rotational speeds which are considerably higher than those of the devices of the prior art, although motors are employed which have less power and a lower cost.
The winding process of the fiber onto the pulley substantially made in the absence of sliding therebetween, is facilitated if the pulley is provided with a suitable profile. In a first embodiment, the pulley is provided with a substantially V-shaped groove, adapted to receive the optical fiber, which comprises opposite side walls forming an angle xcfx86 in the range between 65xc2x0 and 75xc2x0 with the plane of symmetry xcfx80 of the pulley.
In a particularly advantageous embodiment, the groove comprises opposite side walls having a first radially outer portion forming an angle xcfx861 in the range between 65xc2x0 and 75xc2x0 with the plane of symmetry xcfx80 of the pulley and a second radially inner portion forming an angle xcfx862 in the range between 25xc2x0 and 35xc2x0 with said plane of symmetry xcfx80. Preferably, said first and second portions of the side walls are reciprocally connected by means of an intermediate portion having a radius of curvature in the range between 0 and 2 mm. Such a profile advantageously allows to facilitate the winding and unwinding steps of the fiber onto and from the pulley, respectively.
Preferably, the groove comprises a bottom surface which is essentially planar, or anyway having a radius of curvature considerably larger than the radius of the optical fiber, in such a way that the contact between the fiber and the bottom surface and one of the side walls, respectively, is essentially reduced to a point in cross section. If the fiber is wound up onto the pulley for an angle substantially equal to 360xc2x0, the bottom surface of the groove will also have a width adapted to allow for the simultaneous housing of two fiber portions lying side-by-side (respectively associated to a new portion of fiber entering into the pulley and to the portion of fiber leaving the pulley).
Preferably, the pulley is made of a suitable material adapted to develop with the optical fiber a coefficient of friction capable to assure a substantial absence of sliding during the winding up of the fiber onto the pulley. Even more preferably, such coefficient of friction is greater than about 0.4. In this way, it is possible to carry out a fiber twisting without compromising its properties of mechanical resistance, even for values of drawing force smaller than those applicable in the devices of the prior art.
In a third aspect thereof, the present invention relates to an apparatus for manufacturing an optical fiber from a preform, comprising:
means for heating at least one end portion of said preform;
means for drawing an optical fiber from a free end of said heated end portion along a fiber drawing axis;
at least one coating station of said optical fiber; characterized in that it comprises, downstream said at least one coating station, a device of the type described hereinabove.